Ordering Process and Dynamics of "Ceramics"

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"Ceramics" are not only popular and familiar materials in our daily life but also essentialy important one for the modern frontier science and technology. Strucuturally, it is composed of grains of a single crystal which are strongly coupled to each other through an intermediary random surface. "Regularity" and "irregularity" coexist with different geometrical scales in such a material, the essense of which is characterized by a heterogeneous hierarchical structure. In order to study the ordering process, it is essential to examine the dynamical aspect of so called mesoscopic clusters of 10^<-2 3>A.We have, therefore, developed an high-sensitive observation system of nonlinear magnetic responses to an excitation AC field of a wide frequency range from sub-mili up to kiro Hz and of a wide range of amplitude variable from mili to kiro Oe. Then, we have applied it to oxide high-Tc superconductors, graphite intercalation compounds and so on and studied the ordering processes and the dynamical aspects. The experimental results on CoCl_2-GIC and NiCl_2-GIC revealed that nonlinear response is an efficient probe to investigate the nature of hierarchical successive transitions and the dynamical structure os intermediate phase in such "ceramics". In our study, the hierarchical superconductive transitions from intra-to inter-grain direction was first well distinguished separately. The nonlinear response was then found to show different singularitioes at these transition points. From the facts, nonlinear susceptibility has been proved to be an important probe which distinguishes a true superconductive glass from a so called gauge glass and thus to be an essential physical quantity to look after the nature of Josephson-type interactions or the microscopic origin of superconductivity.